Nowadays, voice telephony services in a mobile phone terminal and a mobile network using the 3G W-CDMA (Code Division Multiple Access) technology, are being implemented using a mobile circuit switched network and a line switching protocol.
On the other hand, with researches and development of high-speed and large-capacity technology for mobile networks, the future trend of mobile networks is towards achieving higher speed and larger capacity on the basis of the IP (Internet Protocol), and also towards introducing LTE (Long Term Evolution) and EPC (Evolved Packet Core) as mobile broadband networks. It may be predicted that an existing voice telephony which makes use of a line switching network, will evolve in future to VoIP (Voice over IP) that makes use of IP-based networks.
As a gateway apparatus, reference may be made to, for example, the following Patent Documents 1 to 3. Patent Document 1 discloses a gateway apparatus which is connected between a mobile communication network and an IP network and which may adaptively change a gateway function under a command from a service management apparatus and perform protocol conversion depending on a service class of an application. Patent Document 2 discloses a configuration of a gateway apparatus which is connected between a line switching network and an IP network and which if a display size on a terminal on the line switching network is not coincident with one on the IP network side, a transcoder comes into operation to convert a hit stream. Patent Document 3 discloses a configuration of a gateway apparatus which is arranged between an IP network and a line switching network and which includes a means for transmitting a connection target information message, indicating that the connection target is a television telephone terminal for a switched network, to a television telephone terminal for IP in response to a call request for call between the television telephone terminal for IP and the television telephone terminal for the switched network. In none of these Patent Documents, there is disclosed a gateway apparatus in which a line switching protocol for audio is converted and is transmitted to a mobile IMS network.
[Patent Document 1]
WO2003/107611
[Patent Document 2]
JP Patent Kokai Publication No. JP-P2006-165935A
[Patent Document 3]
JP Patent Kokai Publication No. JP-P2008-42767A